A multilayer brush, in particular a multilayer carbon brush, of the type defined in the introduction is known in general as part of a DC motor. Such a DC motor is mechanically commutated and is used, for example, as an engine starter for an internal combustion engine. An applied current is fed into a rotor winding via a commutator with the aid of one or more brush pairs. The brushes of the brush pairs are usually made of a sintered material, containing essentially copper and graphite components. Both the brushes and the commutator are subject to wear during operation of the engine. The engine starter is typically designed for short-term operation per engine start and is suitable for 30,000 to 60,000 operating cycles.
If the engine starter is to be designed for higher loads and/or longer operating times, in particular with a 12V operating voltage for passenger vehicles, two-layer brushes may be used, for example, where two different carbon brushes are pressed together to form one unit, their layers having a different copper content and having different specific resistivities accordingly. In the combination of the two carbon brush layers, emphasis has previously been placed in particular on maintaining the operating cycles or operating counts with regard to standard operation of the engine starter. In dimensioning the copper components, consequently criteria relating to the power and operating capacity to start the internal combustion engine, as are required in standard operation, come into effect. Predominantly the power of the engine starter and lowering the electrical resistance on the brushes are of primary concern, so that only operating counts in the range of 30,000 to 60,000 are achievable with such carbon brushes. The operating cycles are thus in the category of serial applications and are not in the category of special applications such as a start-stop operation.